DNA Structure and Replication Lecture PDF

Genetic material 1902- Walter Sutton- genes are located in chromosomes 1860s- Friedrich Miescher isolated nucleic acids in the cells nuclei Both proteins and DNA are in the nuclei of eukaryotic cells. Which of the two chemical components is the genetic material? Protein is made of 20 di...

Genetic material 1902- Walter Sutton- genes are located in chromosomes 1860s- Friedrich Miescher isolated nucleic acids in the cells nuclei Both proteins and DNA are in the nuclei of eukaryotic cells. Which of the two chemical components is the genetic material? Protein is made of 20 different amino acids DNA is made of 4 different nucleotides 1920- Griffith 1944- Avery, MacLeod and McCarty Proteins: C, H, O, N, and S DNA: C, H, O, N, and P 32P  Radioactive DNA 35S  Radioactive protein 1952- Hershey and Chase  DNA is made of nucleotides Nucleotide s Nucleoside Nucleoside Nucleoside Figure 07.08 Figure 07.11 How long is a polynucleotide chain? In a diploid human cell there are: About 6 x 109 bp (base pairs) of DNA 46 chromosomes Each chromosome has one DNA molecule, so 6 x 109 /46 0.13 x 109 = 130 x 106 bp Figure 07.08 Differences between DNA and RNA DNA RNA Sugar Deoxyribose Ribose Pyrimidines C, T C, U https://www.dailymotion.com/video/xitly  Chargaff G/C=1 A/T=1 A+G/T+C =1 Purines/pyrimidines=1 Rosalind Franklin Maurice Wilkins  Watson and Crick (B) DNA structure Double helix: Two polynucleotide strands wrapped around each other ▪ The helix is right handed ▪ The two polynucleotide chains are antiparallel (If one goes 5’ to 3’ from top to bottom, the other one goes 3’ to 5’) The sugars and phosphates are facing the outside, the nitrogenous bases face the inside of the helix The nitrogenous bases are almost perpendicular to the axis of the helix The two polynucleotide chains interact through hydrogen bonding between the nitrogenous bases ▪ H bonds can be broken ▪ Very specific interactions: A with T through 2 hydrogen bonds Figure 07.17  δ− δ+ δ+ δ−  Watson and Crick (B) DNA structure There are 10 base pairs (bp) per turn of a helix There are 3.4 nm= 34 Å (Angstroms) per turn of a helix Turn of the helix  Differences between DNA and RNA DNA RNA Sugar Deoxyribose Ribose Pyrimidines C, T C, U Double Single stranded stranded Secondary structure Loop Hairpin Figure 07.18  Alternative nucleic acid structures B structure A structure Z structure (Watson and Crick) orientation Right handed Right handed Left handed Base pairs/ 10 11 12 turn of helix Most common of DNA-RNA hybrids Certain DNA DNA in cells Double stranded- sequences RNA Both strands of DNA are copied all along  Three possible modes of replication 1958- Meselson and Stahl experiment Old DNA, heavy DNA made with 15N New DNA, light DNA made with 14N 1. Semiconservative replication  Three possible modes of replication Old DNA, heavy DNA made with 15N New DNA, light DNA made with 14N 2. Conservative replication  Three possible modes of replication Old DNA, heavy DNA made with 15N New DNA, light DNA made with 14N 3. Dispersive replication  heavy heavy heavy intermediate heavy light intermediate intermediate light heavy light intermediate 1958- Meselson and Stahl experiment DNA Replication is semiconservative in bacteria DNA Replication is semiconservative in eukaryotic cells  DNA replication starts at specific sequences called origins of replication Eukaryotic cell Bacterial cell One single relatively small circular double stranded DNA molecule of genomic DNA One origin of replication Many larger double stranded linear DNA molecules in the nucleus (one DNA molecule per chromosome) Multiple origins of replication per chromosome DNA replication is bidirectional DNA polymerases-enzymes that synthesize DNA Require a template to copy Require a primer with a 3’-OH group Require dNTPS as substrates Only synthesize in the 5’ 3’ direction  ? The DNA at the ends of chromosomes (telomeres) is synthesized in a different way by an enzyme called telomerase, which is present in some cells and prevents the shortening of the DNA molecule in a chromosome in each round of replication. Actually, although the 5’ to 3’ direction of the new strands is opposite, one of the strands is wrapped around, so DNA polymerase moves in the same physical orientation to copy both strands as shown in : https://www.youtube.com/watch?v=gzLqP5_4T-o